Developing high performance n‐type thermoelectric (TE) materials is fundamentally important for developing high efficiency TE devices. AgBiSe2, which reveals superior n‐type TE performance in a cubic phase, crystallizes in a hexagonal phase at room temperature, and typically, undergoes phase transitions to a cubic phase at a temperature above 580 K. Here, for the first time, through entropy optimization with lead‐selenides (≥9.9 mol%), the high‐temperature cubic phase of AgBiSe2 is stabilized from 300 to 800 K. Furthermore, the AgBiSe2‐PbSe pseudo‐binary diagram is established. The resultant alloys with optimized entropy possess unique local distorted cubic lattices, which contribute low lattice thermal conductivity approaching 0.3 W m−1 K−1 in extended operating temperature range. Consequently, a peak figure of merit zT value of ≈0.8 at 800 K and a record‐high average zT value of 0.42 for n‐type I‐V‐VI2 compounds are attained in pure phase cubic n‐type (AgBiSe2)1−x(PbSe)x solid solutions. These results pave the way for developing new TE materials via entropy engineering.

A combination of high yield strength (1.1 GPa) and large tensile elongation to failure (28%) is achieved in a HfNbTiV refractory high-entropy alloy by creating modulated nanoscale inhomogeneity in both composition and lattice strain.

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